ABSTRACT Mature HIV epidemics in high prevalence regions are characterized by significant HIV discordance rates in stable relationships, representing considerable HIV transmission risk. The pandemic primarily affects those in peak years of childbearing, who may not be aware of their HIV status or use condoms. Intentional childbearing may place heterosexual couples at risk of HIV infection in resource-limited settings with high HIV prevalence areas where society places great value on having children. We propose a community-based prospective cohort study that will offer home-based HIV counseling and testing to heterosexually active couples and obtain reproductive and sexual health data, and pregnancy and HIV incidence from men and women in Nyando district, western Kenya. A randomly-selected cohort of HIV discordant (n?960), concordant-negative (subset of n?960) and concordant-positive (n?480) couples will be followed. Aim 1. Determine and compare HIV incidence among HIV-discordant couples who intend pregnancy, versus those who do not intend to become pregnant, using rapid HIV assays every 6 months for 2 years. Aim 2. Describe and compare sexual behaviors, contraception, condom use, couple-level behaviors and factors, and sociocultural norms in the cohort, using rapid urine pregnancy tests, and audio computer-assisted self interviews from each member of the dyad, every 6 months for 2 years. Aim 3. Develop a theoretical sociobehavioral model of how reproductive decision-making processes influence the risk of HIV transmission. This model will be developed using Aims 1 and 2 data as well as qualitative data collected via dyad interviews (n=30) and 12 focus group discussions (among HIV-negative males, HIV-negative females, HIV-positive males, HIV-positive females), conducted at baseline and at month 18. We will apply the theoretical model elements to the cohort data using structural equation modeling to assess its value in predicting incident HIV and pregnancy. A policy meeting will be held in Kenya to discuss study findings. This study will contribute significantly to design of interventions to reduce horizontal and vertical HIV transmission in resource-limited settings. Project Narrative This research addresses an under-studied and urgent global public health issue: improved understanding of HIV and reproductive health behaviors in order to reduce HIV transmission. The study is methodologically innovative as it integrates sociobehavioural and epidemiological data in a prospective, community-based design. Findings have the potential to inform improved, integrated delivery of HIV and reproductive services to heterosexual couples in resource-limited settings with high HIV burden and generalized epidemics.